The present invention relates to a device for detecting that a rotary cutter such as a drill, a tap or an end mill is in an abnormal state and is likely to break unless its rotation is stopped, and for detecting that the end of the life of the cutter is approaching.
As the technology of production advances, it is increasingly demanded that less manpower or no manpower be used in operating machine tools. In order to achieve a semi-automatic or a full automatic machining an in-process detection of tool abnormality such as damage to, or breakage of, a cutting tool must be performed.
An in-process detection method is known which is effective in detecting tool abnormality such as damage to, or breakage of, a rotary cutter. The method comprises the steps of detecting an acoustic emission from a rotary cutter which occurs when the rotary cutter is damaged or broken, generating a signal representing the acoustic emission, and giving an alarm according to the signal. The signal has so high a level that it can be used in the in-process detection of such tool abnormality. The method is disadvantageous, however. The tool abnormality cannot be detected before the tool is damaged or broken. If a rotary cutter is broken while cutting a workpiece, a fragment remains in the workpiece. In order to further cut the workpiece, the fragment must be pulled out, which would take much time and labor. What is more, the fragment may damage the workpiece as it is pulled out, thus leaving a defective product. Consequently, the yield of products may be reduced and the cost of products may rise.
Another in-process detection method is known for predicting the damage to, or breakage of, a drill. The method utilizes a variation in load current of a motor which rotates the drill. The variation of load current is, however, small when the drill has a diameter of 7 mm or less. The method does not well work in predicting the damage to, or breakage of, a drill which is 7 mm or less thick.
Still another in-process detection method is proposed for predicting the damage to, or breakage of, a drill. In this method a strain gauge is attached to a spindle. As the drill is loaded, the spindle driving the drill is gradually strained. The strain gauge detects the strain of the spindle, thereby predicting the damage to, or breakage of, a drill. The load applied on a drill of a small diameter is small, and the method cannot precisely predict the damage to, or breakage of, a drill of a small diameter.
As mentioned above, no method has been proposed that is effective in predicting and thus preventing the damage to, or breakage of, a rotary cutter, particularly a drill having a small diameter. If an automatic machining is to be performed using a machine tool such as a machining center, use of small-diameter rotary cutters has to be avoided.